Revision notes · Homeostasis and response
The human nervous system
Structure and function4.5.2.1
Definition: The central nervous system consists of the brain and spinal cord, which process information from receptors and coordinate responses.
The human nervous system detects stimuli from the environment and coordinates rapid electrical responses.
- •Information travels along neurones as electrical impulses from receptors to the central nervous system and then to effectors.
- •A reflex arc is an automatic, rapid pathway that bypasses conscious parts of the brain to protect the body from harm.
- •The reflex path is: stimulus -> receptor -> sensory neurone -> relay neurone -> motor neurone -> effector -> response.
- •At a synapse, electrical impulses trigger chemical neurotransmitters to diffuse across the gap between neurones.
- •Reaction time is the time taken to respond to a stimulus and can be measured experimentally.
⚠️ Common mistake: Thinking that reflex actions involve conscious thought from the brain, rather than passing through the spinal cord or unconscious brain regions.
🧠 Remember: Reflex arc order: Some Red Snakes Run Much Easier (Stimulus, Receptor, Sensory, Relay, Motor, Effector).
The brain4.5.2.2
Definition: The brain is a complex organ containing billions of interconnected neurones that controls complex behaviour and body functions.
Different regions of the brain carry out specific functions necessary for consciousness, movement, and survival.
| Region | Function |
|---|---|
| Cerebral cortex | Consciousness, intelligence, memory, language |
| Cerebellum | Muscle coordination, movement and balance |
| Medulla | Unconscious activities (e.g. heart rate, breathing rate) |
- •Scientists map brain function by studying patients with brain damage, using electrical stimulation, and taking MRI scans.
- •Treating brain disorders is difficult due to the complexity, delicate nature, and risk of permanent damage to brain tissue.
⚠️ Common mistake: Confusing the cerebellum (controls balance and movement) with the cerebral cortex (controls high-level conscious processes like memory and thought).
🧠 Remember: Cerebel-balance (cerebellum controls movement and balance) and Medulla-maintain (medulla maintains vital unconscious processes).
The eye4.5.2.3
Definition: The eye is a sensory organ containing receptors sensitive to light intensity and colour.
The eye changes its structure to adjust focus for distance and control the amount of light entering.
- •Light passes through the cornea and pupil before being focused by the lens onto the retina.
- •Accommodation is the process of changing the shape of the lens to focus on near or distant objects.
- •Vision defects include myopia (short-sightedness, corrected with concave lenses) and hyperopia (long-sightedness, corrected with convex lenses).
| Near objects | Distant objects | |
|---|---|---|
| Ciliary muscles | Contract | Relax |
| Suspensory ligaments | Loosen | Pull tight |
| Lens shape | Thick and curved | Thin |
⚠️ Common mistake: Stating that suspensory ligaments contract or relax; only muscles contract, so ligaments can only be pulled tight or go slack.
🧠 Remember: Near objects make Ciliary muscles Contract (both start with C).
Control of body temperature4.5.2.4
Definition: Thermoregulation is the process of maintaining core body temperature near 37 °C to optimize enzyme function.
Body temperature is monitored and controlled by the thermoregulatory centre in the brain.
- •The thermoregulatory centre in the brain contains receptors sensitive to blood temperature and receives impulses from skin receptors.
- •When too cold, erector muscles in the skin contract to raise body hairs, trapping an insulating layer of air near the skin to reduce heat loss.
| Too hot | Too cold |
|---|---|
| Vasodilation — blood vessels near the skin widen, increasing heat loss | Vasoconstriction — blood vessels near the skin narrow, reducing heat loss |
| Sweat glands produce sweat, which evaporates to cool the skin | Shivering — skeletal muscles contract rapidly, increasing respiration to release heat |
⚠️ Common mistake: Thinking that blood vessels move closer to or further from the skin surface, rather than widening (vasodilation) or narrowing (vasoconstriction).
🧠 Remember: Vasodilation makes vessels Dilated (wider) to drop heat; Vasoconstriction makes vessels Constricted (narrower) to catch heat.
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